The present invention relates to model steam locomotives or railway engines and, more particularly, to valve gear reproduced in such models.
An essential element of a real steam locomotive is the valve gear, which is a gear designed to control the admission and discharge of steam to and from the cylinders of the locomotive. One of the preferred types of valve gear is known as the Walscheart's valve gear.
On a real steam locomotive, the valve gear is arranged to be under the control of the driver who, by altering its configuration, adjusts the pulling power of the locomotive to the load. For completeness, it should be added that there are other factors involved which also enable the power of a locomotive to be adjusted in accordance with the load that it has to pull, among which are the boiler pressure and the regulator opening.
In models of locomotives fitted with valve gear, a considerable increase in realism is obtained by modelling this valve gear. In the case of a model, there being no driver, generally no attempt has been made to construct the valve gear so that it alters its configuration in a realistic manner according to the load being pulled by the locomotive, although some attempt has been made in the case of model locomotives actually driven by steam. This invention, however, relates to model locomotives driven, in a manner which is now commonplace, by electric motors connected either to the driving wheels of the model locomotive or other subsidiary wheels. In this case, as there is no functional need to alter the configuration of the valve gear and the scale of the locomotive is usually such that small mechanisms are difficult to alter, it is customary to model the valve gear in a single, set configuration.
The specific alteration of the configuration in a steam locomotive fitted, for example, with Walscheart's valve gear, which was visible to a bystander, was the alteration of the radius rod with respect to the expansion link. The radius rod was provided with a degree of radial travel such that the ability of the locomotive to do work could be smoothly altered from a position which produced maximum effort in a forward direction to a position which produced maximum effort in a reverse direction. Most intermediate positions were also available including a mid-position which prevented the locomotive from exerting any effort in either direction. In modelling Walscheart's valve gear in locomotives available on the market, the radius rod is almost invariably positioned in the mid-position such that the locomotive is apparently configured for producing no effort in either direction.
It has hitherto been known to construct scale model steam locomotives powered by electric motors with valve gear which can be configured from a certain forward motion position to another certain backwards motion position by means of a device known as a slip eccentric mounted on a convenient axle. By means of this device, the valve gear is configured from one of the two available positions to the other each time the locomotive changes its direction of travel. It is recognised that this gives an increased degree of realism over a locomotive fitted with valve gear fixed in the mid-gear-position, but it is still not a true reproduction of the full scale gear.